A study of the controlled degradation of polypropylene containing pro-oxidant agents
© de Carvalho et al.; licensee Springer. 2013
Received: 31 July 2013
Accepted: 9 October 2013
Published: 20 November 2013
Intentional degradation by pro-oxidant agents, many of which are metal-based, can result in uncertainty as to the time of biodegradation. Polyacetal (POM) is a thermoplastic polymer commercially classified as an engineering polymer and contains carbon, hydrogen and oxygen. The depolymerization of POM during processing can enhance thermal decomposition. The aim of this study was to investigate the controlled degradation of polypropylene induced by the degradation of POM or d2w®. Mixtures of polypropylene containing different concentrations of POM or d2w® were prepared by extrusion. The properties of the mixtures (blends) were evaluated based on the melt index (MFI), tensile properties, Fourier transform infrared spectroscopy (FTIR), Time inductive oxidation (OIT) and Thermogravimetric analysis (TGA). The two additives (POM and d2w®) enhanced the oxidative thermal degradation of polypropylene and the degradation of the polypropylene/POM mixture could be controlled by altering the POM concentration.
Packaging waste accounted for 78.81 million tons or 31.6% of municipal solid waste (MSW) in the United States in 2003, 56.3 million tons or 25% of MSW in Europe in 2005, and 3.3 million tons or 10% of MSW in Australia in 2004. In the US, the predominant method of waste disposal is currently landfill packaging, followed by recycling, composting and incineration (Kale et al. 2007). Commodity polymers (polyethylene PE, polypropylene PP, polystyrene PS, polyvinyl chloride PVC and polyethylene terephtalate PET) prevail in packaging applications (PlasticsEurope 2011) and polyolefins are increasingly being used in new applications (Gahleitner 2011). An excellent way of producing degradable polyethylene is to mix this polymer with pro-oxidant additives that can effectively improve the degradability of these materials (Roy et al. 2007). Intentional degradation by pro-oxidant agents, many of which are metal-based (Roy et al. 2007), has generated uncertainties in the evaluation of biodegradation (European Bioplastics 2012) and several surveys it is claimed that polyolefins (PE, PP) is an inert polymer with good resistance to microorganisms (Albertsson 1978 2003). The controlled degradation of polypropylene has been used in rheological control by distributing and reducing the molar mass of organic peroxides in reactive extrusion (Rocha et al. 1994; Kim 1996). Polyacetal (POM) is a thermoplastic polymer that is susceptible to thermal decomposition (depolymerization) (Cottin et al. 2000). The objective of this study was to investigate the controlled degradation of polypropylene induced by the degradation of an organic oxidizing agent (POM) in extrusion. The additive d2w®, a commercial metal-based pro-oxidant, was used for comparison.
Materials and methods
Isotactic polypropylene (iPP) H603 (density: 0.905 g/cm3; MFI: 1.5 g/10 min) was used in granulated form, as supplied by Braskem (Triunfo, RS, Brazil). The polyacetal copolymer (density: 1.42 g/cm3; MFI: 14.0 g/10 min) was used in powder form as supplied by Ticona (São Paulo, SP, Brazil). The commercial pro-oxidant additive d2w® was supplied by RES Brazil (São Paulo, SP, Brazil).
Preparation of the mixtures
The polymer/additive mixtures used in this study
Melt flow index (MFI)
The MFI was determined in a plastometer (model 7023.000, CEAST, Ohio, USA) according to ASTM D-1238 (ASTM 2004). The test conditions were set at a load of 2,160 kg and a temperature of 230°C for all mixtures.
Type IV specimens (ASTM D-638-10) (ASTM 2010) were injected into a model PIC-BOY 22 machine (Petersen & Cia Ltda, São Paulo, SP, Brazil) with an injection capacity of 22 g of polystyrene. The total cycle time was 30 s and the temperatures of zones 1 (injection nozzle), 2 and 3 were 220°C, 220°C and 180°C, respectively. The tensile test was done in a universal testing machine (model 5569, Instron), according to ASTM D638-10, at a test speed of 25 mm/min and cell load of 50 kN. The tensile strength at break and elastic modulus were determined.
Fourier transform infrared spectroscopy (FTIR)
Films 30 ± 2 μm thick were prepared at 190°C with a compression pressure of 2000 psi and compression time of 80 s. FTIR measurements were obtained using a Varian 660-IR FT-IR spectrometer operated in transmittance mode. Thirty-two scans were obtained in triplicate from 4000 cm-1 to 400 cm-1 at a resolution of 4 cm-1. The influence of POM and d2w® on polypropylene oxidation was determined from the spectra by calculating the carbonyl (CI) and hydroxyl (HI) indices based on the relationships CI = A1725/A2722 and HI = A3500/A2722, respectively.
Differential scanning calorimetry (DSC)
DSC was done in a TA Instruments calorimeter at a nitrogen flow of 50 ml/min. Approximately 10 mg of each sample was heated, cooled and heated again over a temperature range of 25–250°C at a heating and cooling rate of 10°C/min. The melting temperature (Tm), crystallization temperature (Tc) and degree of crystallinity were calculated using the enthalpy of fusion values of 209 J.g-1 and 306 J.g-1 for 100% crystalline polypropylene and polyacetal, respectively (Canevarolo 2003; Kumar et al. 1995).
Oxidation induction time (OIT)
The OIT was determined by exposing ~10 mg of each mixture to a nitrogen flow of 50 ml/min and a heating rate of 20°C/min. An oxygen flow of 50 ml/min was used after melting at 200°C.
Thermogravimetric analysis (TGA)
TGA was done in equipment from TA Instruments. Approximately 10 mg of each mixture was placed in a nitrogen atmosphere and heated at a rate of 10°C/min over a temperature range of 25–550°C. The nitrogen flow over the measurement cell was 50 ml/min. The activation energy of degradation (Ea) was determined according to ASTM E1641 (ASTM 2007).
Results and discussion
Melt flow index (MFI)
Infrared spectroscopy (FTIR)
Differential scanning calorimetry (DSC)
Thermal properties of pure materials (PP 1 , POM and d 2 w®) and blends of PP/POM and PP/d 2 w®
Oxidation induction time (OIT)
Oxidation induction time for pure materials and blends
Pure materials and blends
In PP/POM blends, there was a marked decrease in the OIT values from blend PP6 onwards. In contrast, there was an increase in the OIT values of blends PP2 and PP4, i.e., a stabilizing (antioxidant) effect. A similar delay in the kinetics of degradation was observed in the absence of oxygen in the TGA of these two blends, i.e., the Ti of the blends was greater than that of pure polypropylene (PP1). The OIT and TGA results indicated that d2w® concentrations ≥2% increased the thermal stability of the blends. In the case of POM, there was a decrease in the stabilizing synergistic effect at concentrations up to 3%; at higher concentrations, POM had an oxidizing effect on polypropylene.
The oxidation of a polymer involves a complex chain of reactions that involves many steps such that the overall Ea is the sum of the energies of activation of individual stages. In this chain of reactions there may be temperature ranges in which deviations from Arrhenius’ law can be neglected, e.g., with blends PP2 and PP4. The oxidation of polypropylene (in powder form) has been referred to as non-homogeneous (heterogeneous) kinetics that is characterized by chemiluminescence (Celina and George 1995). This oxidation is based on a model containing small numbers of localized zones (amorphous regions) in which oxidation occurs at a high rate and from where it spreads to other regions. The presence of stabilizers retards the diffusion of volatile degradation products for a short period of time known as the induction period. Even using sensitive techniques involving photon emission, such as chemiluminescence, the investigation of this phenomenon over such a short timescale is a difficult task, even though the Ea is higher in this period (Celina and George 1993). Several studies (Wang et al. 2011; Bouhelal et al. 2010; Groening and Hakkarainen 2002; Albertsson and Hakkarainen 2008) have shown that the decomposition of hydroperoxides in polypropylene leads to the formation of volatile products and that water is a major product of degradation but does not interfere with the spread of oxidation. During this period, generally only a decrease in polymer molecular mass is observed, along with the formation of volatile, low molecular mass products. Eriksson (Eriksson et al. 2001) suggested that following the formation of peracids by the oxidation of formaldehyde, the spreading of oxidation is favored by the gas phase and that the relatively low reactivity of formaldehyde allows greater diffusion to more distant regions.
Thermogravimetric analysis (TGA)
Together, these results indicate that the addition of d2w® to polypropylene increased the thermal stability in a manner dependent on the concentration of additive. The low production of volatile components and the reduced mobility (diffusion capacity) of the polypropylene matrix were probably important factors in this thermal stability. The addition of POM to polypropylene resulted in synergistic and antagonistic effects (stabilization and degradation), with the extent of stabilization and degradation depending on the amount of volatile components produced.
Activation energy (Ea) and correlation coefficients for pure materials and blends
The addition of POM or d2w® promoted the oxidative thermal degradation of polypropylene (PP), with the extent of degradation being regulated by the POM concentration in PP/POM blends. At concentrations <3% (w/w), POM enhanced the thermal stabilization of polypropylene under the conditions investigated, whereas at concentrations >3% POM stimulated the oxidation of polypropylene. These results suggest that the POM with a concentration >3% (w/w), may act as a pro-oxidant agent of the PP, and the synergistic effect of degradation can be maximized by increasing the miscibility at the interface of the blend PP/POM.
The authors thank UFABC and CAPES for financial support and scholarships.
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