Modificação do Poli(hidroxibutirato) PHB com nucleantes e com o copolímero (etileno-co-acetato de vinila) - EVA
Abstract
In this work, we sought to modify PHB, processing it in the melted state, and to
evaluate the effect of the modifications on its mechanical properties and
biodegradation characteristics. The following systems were therefore studied: a
PHB/EVA blend, in a 90/10 ratio, with an ethylene-co-vinyl acetate copolymer
containing 28% of VA; a PHB and PHB/EVA blend with 1% of Boron Nitride
(BN) nucleant; a PHB and PHB/EVA blend with 0,35% of HPN nucleant
( Hyperform Nucleant ); and a PHB/EVA blend with 5% of Elvaloy PTW, which
was used as a compatibilizer. These systems were characterized by thermal,
mechanical, morphological and biodegradation analyses using the following
techniques: Melt Flow Index (MFI), Differential Scanning Calorimetry (DSC),
Thermogravimetry (TG), Dynamic Mechanical Thermal Analysis (DMTA), Size
Exclusion Chromatography (SEC), Tensile Mechanical and Izod Impact
(notched) Tests, Wide-Angle X-ray Diffraction (WAXD), Scanning Electron
Microscopy (SEM), and Atomic Force Microscopy (AFM). The biodegradability
test involved burying the samples in a suitable compost for 60, 120 and 180
days. The DCS analysis indicated that Boron Nitride was an effective nucleating
agent for the PHB. However, the HPN did not act as a nucleant on this polymer,
but instead, induced degradation of the PHB upon heating, as indicated by MFI,
DSC and TG. Moreover, the addition of BN or even EVA did not improve the
mechanical properties of PHB, and Elvaloy PTW did not act as a compatibilizing
agent for the PHB/EVA blend. The HPN was found to interact antagonically with
the PHB, which displayed extremely low impact strength values and widely
dispersed results. The SEM and AFM analyses revealed poor distribution and
inadequate dispersion of EVA in the PHB matrix. In addition, the AFM technique
indicated that the PHB topography displayed the presence of pores, which
undoubtedly, combined with the other factors, hindered its modification. Biodegradation was found to be strongly composition-dependent, since the
addition of 10% of EVA to the PHB significantly accelerated the degradation of
the PHB in this blend. In other words, the PHB probably exhibited disordered
chain packing in the proximities of the EVA, facilitating the process of
biodegradation by microorganisms. Hence, this study demonstrated the
difficulty of modifying PHB, since it involves a variety of factors, as well as the
possibility of developing PHB-based materials that biodegrade more or less
rapidly according to the needs of a specific application.