Abstract
The effect of accelerated weathering on poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and PHBV-based nanocomposites with rutile titanium (IV) dioxide (PHBV/TiO2) was investigated. The accelerated weathering test applied consecutive steps of UV irradiation (at 340 nm and 0.76 W m−2 irradiance) and moisture at 50 °C following the ASTM D4329 standard for up to 2000 h of exposure time. The morphology, chemical structure, crystallization, as well as the mechanical and thermal properties were studied. Samples were characterized after 500, 1000, and 2000 h of exposure time. Different degradation mechanisms were proposed to occur during the weathering exposure and were confirmed based on the experimental data. The PHBV surface revealed cracks and increasing roughness with the increasing exposure time, whereas the PHBV/TiO2 nanocomposites showed surface changes only after 2000 h of accelerated weathering. The degradation of neat PHBV under moisture and UV exposure occurred preferentially in the amorphous phase. In contrast, the presence of TiO2 in the nanocomposites retarded this process, but the degradation would occur simultaneously in both the amorphous and crystalline segments of the polymer after long exposure times. The thermal stability, as well as the temperature and rate of crystallization, decreased in the absence of TiO2. TiO2 not only provided UV protection, but also restricted the physical mobility of the polymer chains, acting as a nucleating agent during the crystallization process. It also slowed down the decrease in mechanical properties. The mechanical properties were shown to gradually decrease for the PHBV/TiO2 nanocomposites, whereas a sharp drop was observed for the neat PHBV after an accelerated weathering exposure. Atomic force microscopy (AFM), using the amplitude modulation–frequency modulation (AM–FM) tool, also confirmed the mechanical changes in the surface area of the PHBV and PHBV/TiO2 samples after accelerated weathering exposure. The changes in the physical and chemical properties of PHBV/TiO2 confirm the barrier activity of TiO2 for weathering attack and its retardation of the degradation process.
Highlights
Petroleum-based polymers present substantial environmental problems, in the production process, and at the end of life, because of their slow degradation and harmful degradation products.Biodegradable polymers have the advantage of a low environmental impact and high sustainability [1].Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), one of the most used polyhydroxyalkanoates (PHAs), is a linear, hydrophobic thermoplastic, and a semicrystalline polyester and biodegradablePolymers 2020, 12, 1743; doi:10.3390/polym12081743 www.mdpi.com/journal/polymersPolymers 2020, 12, 1743 polymer
The neat PHBV and the PHBV/TiO2 nanocomposite were placed between two pieces of stainless steel plate covered by a release foil, and molded into 1 mm thick sheets at the same temperature for 5 min using a hydraulic press (Carver, Inc., Wabash, IL, USA) at a pressure of 50 bar
The degradation observed in the Atomic force microscopy (AFM) and Scanning Electron Microscopy (SEM) analyses continued with increasing exposure time, these results suggest a combination of chemical and physical degradation that contributed to the surface properties for neat PHBV
Summary
Petroleum-based polymers present substantial environmental problems, in the production process, and at the end of life, because of their slow degradation and harmful degradation products.Biodegradable polymers have the advantage of a low environmental impact and high sustainability [1].Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), one of the most used polyhydroxyalkanoates (PHAs), is a linear, hydrophobic thermoplastic, and a semicrystalline polyester and biodegradablePolymers 2020, 12, 1743; doi:10.3390/polym12081743 www.mdpi.com/journal/polymersPolymers 2020, 12, 1743 polymer. Biodegradable polymers have the advantage of a low environmental impact and high sustainability [1]. Its physical and chemical properties make it very attractive for several applications, such as a substitute for non-biodegradable polymers in medical uses such as antitumor and vascular system materials, as well as in pharmaceutical applications such as biocompatible drug delivery systems [2,3,4]. PVBV-based materials have been used more and more in high performance food packaging, single-use and disposable items, housewares, electrical and electronics devices, agriculture and soil stabilization, adhesives, paints and coatings, and automotive parts [4,5,6]. Polymer blending is an effective approach to boost some polymer characteristics, tailoring its physico–chemical properties and overcoming the specific limitations of the PHBV. Reinforcement with natural fibers (such as maple wood, bamboo or straw fibers) and nanofillers (such as graphene and derivatives, nanocellulose, nanoclays and nanometals) has been a field of study for researchers searching for the previously cited properties, and looking for the enhancement of the miscibility of the polymer blends [2]
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