Development of Degradable Polypropylene Using Organic Additives

Abstract

Polyolefins like polypropylene (PP) are extensively used for packaging applications because they are economical and convenient. However, PP is not eco-friendly because of its nonbiodegradable nature. Consequently, its disposal and management at the end of the service life are a cause of significant global concern. These factors have led to the development of environmentally degradable polymeric material that can degrade under composting. The main objective of this study was to develop biodegradable PP by compounding with pro-oxidant. Cobalt stearate remained the focus because it exhibited promising results for initiating thermooxidative degradation and hence biodegradation in earlier studies. Before and after abiotic pretreatment (accelerated weathering), the influence of pro-oxidant loading on the molecular weight changes, morphological, physical, thermal, chemical, and biodegradability characteristics of packaging films was studied. Before abiotic pretreatment, the rheological properties of packaging films were also studied. The biodegradation kinetic modelling of oxidants (two different types) loaded PP films without and with abiotic pretreatment was then carried out. The effect of the biodegradation intermediates on the environment has also been evaluated by different ecotoxicity tests. This work has two parts, the first in which CoSt is used, and the second in which modified CoSt is used as pro-oxidant. In the first part of this work, PP filled with different proportions (0 to 2 phr) of CoSt were prepared in a twin-screw extruder by compounding technique. Eight films of these compounds were prepared using compression moulding. Subsequently, abiotic pretreatment (accelerated weathering) was conducted to investigate the effect of accelerated weathering of CoSt filled PP composite films. These films were characterized for molecular weight changes, chemical, physical, thermal, and morphological properties (before and after abiotic and biotic treatments). Rheological properties of the CoSt filled PP composites were investigated. The biodegradation (biotic treatment) of the CoSt filled PP films without and after abiotic pretreatment were studied as per ASTM D 5338 under controlled composting conditions. The biodegradation kinetic modeling of CoSt filled PP films without and after abiotic pretreatment was then carried out. The biodegradation intermediates were subsequently evaluated for their eco-toxicological impact. The presence of CoSt and other oxygen products after degradation of composite PP by abiotic pretreatment was confirmed by Fourier transform infrared spectroscopy. The highest carbonyl index (0.83) was found in the film with 2 phr CoSt sample after abiotic pretreatment. As the addition of CoSt was progressively increased from 0 to 2 phr, the tensile strength (before abiotic pretreatment) decreased from 35 to 14.5 MPa thermal stability also decreased as shown by TGA. Before and after abiotic pretreatment, the crystallinity of CoSt filled PP decreased with increase in CoSt from 0.2 to 2 phr, as indicated by DSC and XRD. Rheological studies confirmed the pseudo plastic nature of all the modified composites. Without and after abiotic pretreatment, the maximum biodegradation by the film having 2 phr CoSt was shown to be 19.80%, and 36.42 % respectively. SEM results of biotically treated films also showed erosion, several small pits and increased roughness on the modified PP films. The surface morphology of the films supported the biodegradation results. The significant decrease in molecular weight proved that chain scission mechanism occurred leading to the formation of intermediates, which further led to microbial assimilation. The overall results indicated that the addition of CoSt and abiotic treatment significantly enhanced the biodegradation of PP. This work was also aimed at modeling the kinetics of biodegradation of PP loaded with CoSt as pro-oxidant without and after abiotic pretreatment. The experimental data were analyzed using an eight-parameter Komilis model containing a flat lag phase. The model formulations involved hydrolysis of primary solid carbon and its subsequent mineralization. The first step was rate controlling and it included hydrolysis of slowly (Cs), moderately (Cm), and readily (Cr) hydrolysable carbon fractions in parallel. The model parameters were evaluated by means of nonlinear regression technique. The model parameters and undegraded/ hydrolysable/mineralizable carbon evolutions followed only one kinetic regime in both without and with abiotic pretreatment. All the films involved moderately, and readily hydrolysable carbons but the absence of slowly hydrolysable carbon. For the films without abiotic pretreatment, the rate of degradation reached its maximum (0.223-0.740 % per day) at around 5-11th day. With abiotic pretreatment, the rate of degradation reached its maximum (0.322-0.897 % per day) at around 12-39th day. For all the films, readily hydrolysable carbon fractions and their hydrolysis rate constants (kr) appreciably increased with increasing prooxidant loading. All the films showed the presence of growth phase because of their high initial readily hydrolysable carbon fractions. The methods presented here can be used for the design and control of other similar systems. The eco-toxicity tests of the degraded material, namely, microbial test, plant growth test, and earthworm acute-toxicity test demonstrated that the biodegradation intermediates were nontoxic. Hence, CoSt filled PP has high industrial potential to make biodegradable flexible packaging. In the second part of this work, the changes in the properties of PP after loading of modified- CoSt pro-oxidant were studied. A master batch containing 10 phr of cobalt stearate in PP (PP100CoSt10) was made in a co-rotating twin-screw extruder at a speed of 150 rpm under a N2 blanket and pelletized. This masterbatch was aged at 110 oC for 48 h. After that, it was crushed into powder, sieved, and used as a modified pro-oxidant (designated as T). Different proportions of modified-CoSt pro-oxidant were filled (5-30 phr) in PP, and composites were prepared. Five films of these composites were prepared using hot press moulding. These films were studied as in the first part. Before and after pretreatment, the carbonyl index of the films increased to (1.1 maximum) with modified pro-oxidant loading and time of abiotic pretreatment. The tensile properties (before abiotic pretreatment) of the modified pro-oxidant loaded PP films decreased. The thermal stability (before and after abiotic pretreatment) of the modified pro-oxidant filled PP films reduced as indicated by TGA. After abiotic pretreatment, the DSC results confirmed that the addition of modified pro-oxidant reduced the crystallinity of the PP films from 63 to 42.53 % (and 77 to 40.5% from XRD data). This led to enhanced degradation of PP. Before abiotic pretreatment, pseudo plastic behavior of all the modified- CoSt loaded PP films was confirmed by rheological studies. The highest biodegradability (28.87%) was obtained in the film containing 30 phr of modified pro-oxidant without abiotic pretreatment. With abiotic pretreatment, the maximum biodegradability (38.78%) was obtained in an abiotically treated film containing 30 phr of modified pro-oxidant in it. After the biodegradability test, SEM results also revealed several small pits, erosion, as well as enhanced roughness on the PP film samples. Decrease of molecular weight is higher in the case of modified pro-oxidant compared with CoSt pro-oxidant. The kinetic modeling of the biodegradation was carried out in the same manner as was done above in the first part. The same Komilis model with a flat lag phase was used to correlate the experimental data. The model parameters and un-degraded/hydrolysable/mineralizable carbon evolutions followed only one kinetic regime in both before and after abiotic pretreatment. All the films involved moderately, and readily hydrolysable carbons but the absence of slowly hydrolysable carbon. For the films without abiotic pretreatment, the rate of degradation reached its maximum (0.407-0.730 % per day) at around 18-23th day. With abiotic pretreatment, the rate of degradation reached its maximum (0.408-0.920) % per day) at around 15-25th day. For all the films, readily hydrolysable carbon fractions and their hydrolysis rate constants (kr) appreciably increased with increasing modified pro-oxidant loading. All the films showed the presence of growth phase because of their high initial readily hydrolysable carbon fractions. All the eco-toxicity tests of the degraded product materials demonstrated that the degraded products were nontoxic. Hence, the prepared composites can be effectively used as biodegradable flexible packaging materials. A schematic of the overall thesis work is shown in Fig. 1.