Development of Degradable Polypropylene by Radiation Grafting and Blending with Polylactic Acid

Abstract

Simultaneous radiation grafting was optimized to graft acrylic acid monomer on the polypropylene (PP) films to make them hydrophilic and enhance their biodegradability. Experiments were designed based on full factorial central composite design (response surface methodology) and influence of monomer concentration, radiation dose, inhibitor concentration, sulfuric acid concentration on degree of grafting was investigated. The extent of grafting was found to increase with increasing monomer concentration, inhibitor concentration and radiation dose. Different degrees of grafted PP were used for different applications. 35% grafted PP was chosen as our optimum grafted material due to desirable tensile strength (above 20 MPa) for packaging application. The targeted 35% grafting could be achieved at the optimum conditions - monomer concentration 12.09 wt%, radiation dose 12.40 kGy, inhibitor concentration 0.07 M and sulfuric acid concentration 0.12 M. The grafted PP films at different degrees of grafting were tested for tensile properties and characterized by swelling test, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Successful grafting of acrylic acid onto PP films was indicated by FTIR and confirmed quantitatively by determination of carboxylic groups on the film surface. Tensile strength of grafted PP films decreased with increase in degree of grafting. The crystallinity of the grafted films was lower than that of PP film as indicated by DSC studies. Grafting of acrylic acid increased the roughness on the surface of PP films indicated by SEM studies. Thermal stability and degradation behavior of acrylic acid grafted polypropylene (PP-g-AAc) films were investigated by using thermogravimetric analysis (TGA) at four different heating rates 5, 10, 15 and 20 °C/min over a temperature range of 40 to 550 °C in nitrogen atmosphere. The kinetic parameters namely activation energy (Ea), reaction order (n) and frequency factor (Z) were calculated by three multiple heating rate methods. The thermal stability of PP-g-AAc films is found to decrease with increase in degree of grafting. The TGA data and thermal kinetic parameters were also used to predict the lifetime of grafted PP films. The estimated lifetime of neat PP as well as grafted PP decreased with increase in temperature by all the three methods. Studies also indicated that Ea and lifetime of PP-g-AAc films decreased with increase in degree of grafting, which may also be helpful in biodegradation of grafted PP films. The maximum biodegradability of the 35% grafted film was ~ 6%. Residue after biodegradation of grafted samples were evaluated for ecotoxicological impact by microbes and plants (corn and tomato) growth test as per OECD 208 guidelines. Ecotoxicological test indicated that biodegradation intermediates were non-toxic in nature. Melt blending technique was used to prepare blends of PP and PLA with or without compatibilizer, and pro-oxidant and composites of PP and compatibilized blend with nanoclay. Compression molding was used to prepare the films of blend and composite materials. The optimum weight of pro-oxidants and nanoclay was selected 0.2 and 2 wt% on the basis of tensile test results. Blends PP85PL15 and PP85PL15MA4 are the optimum from tensile strength point of view. CoSt and CaSt (0.2 wt%) were added separately to PP85PL15MA4 blend. Nanoclay was added separately to PP85PL15MA4 blend in 2 wt%. The optimized blends and composites were further characterized by FTIR, TGA, DSC, XRD, rheological studies, SEM, biodegradability test and ecotoxicological evaluation. Pseudo-plastic nature of all the blended and composite films was confirmed by rheological studies. Thermal stability of polypropylene films decreased with addition of polylactic acid as confirmed by TGA analysis. The thermal degradation kinetic parameters namely activation energy, order of reaction and frequency factor of the samples were determined over a temperature range of 30 to 550 °C under nitrogen atmosphere at four different heating rates (5, 10, 15 and 20 °C/min). The activation energy was calculated by Kissinger, Kim-Park and Flynn-Wall methods. The activation energy value of PP was much higher than that of PLA. Whereas, addition of polylactic acid, pro-oxidant and nanoclay in PP decreased the activation energy. Addition of compatibilizer increased the compatibility and activation energy of blended films upto some extent. The lifetime of PP was found to decrease with addition of polylactic acid, pro-oxidant and nanoclay. This study indicated that the thermal degradation behavior and lifetime of the investigated samples depend on the fractions of constituents and heating rates. The maximum biodegradation of around 9% could be achieved for PP85PL15MA4CoSt0.2 blend. Biodegraded intermediates were non-toxic in nature as confirmed by ecotoxicological test.