Effects of Rutile–TiO2 Nanoparticles on Accelerated Weathering Degradation of Poly(Lactic Acid)

Abstract

The effect of accelerated weathering on poly(lactic acid) (PLA) and a PLA nanocomposite with rutile titanium (IV) dioxide (rutile–TiO2) was investigated. The accelerated weathering test applied consecutive steps of ultraviolet (UV) (at 340 nm and 0.76 W m−2 irradiance) and moisture at 50 °C for 2000 h, following the ASTM D4329 standard. The morphology, chemical structure, molecular weight, crystallization, as well as mechanical and thermal properties were thoroughly studied. Samples were characterized after 500 h, 1000 h and 2000 h exposure. Different degradation mechanisms were proposed to happen during the weathering exposure and confirmed based on the experimental data. The PLA and PLA/TiO2 surfaces presented holes and increasing roughness over the exposure time. The molecular weight of the weathered samples decreased due to chain scission during the degradation processes. Thermal stability decreased in the presence of TiO2 and a double melting peak was observed for the PLA/TiO2 nanocomposite. A general improvement in the mechanical properties of the PLA/TiO2 nanocomposite was observed over time during the accelerated weathering analysis up to 1000 h of exposure time. After 2000 h of weathering exposure, the PLA and PLA/TiO2 became extremely brittle and lost their ductile properties. This was ascribed to a significant increase in the degree of crystallinity upon weathering, which was accelerated in the presence of TiO2. Atomic force microscopy (AFM) using amplitude modulation–frequency modulation (AM–FM) tool confirmed the mechanical changes in the surface area of the PLA samples after accelerated weathering exposure. The stiffness and Young’s modulus achieved higher values than the unweathered ones up to 1000 h of exposure time. The changes in the physical and chemical properties of PLA/TiO2 over the ageing time confirm the photocatalytic activity of rutile–TiO2.

Accelerated Weathering Effects on Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and PHBV/TiO2 Nanocomposites

“…The T g values did not change with increasing weathering time for the nanocomposite, while there was an increase in these values with increasing weathering time for neat PHBV. This suggests that the smaller crystallites, formed during UV-initiated degradation, restrained the chain movement so that higher temperatures were needed for the amorphous chains to become mobile [ 70 , 71 ].…”

“…However, a recent study reported that the photodegradation of poly(butylene succinate- co -butylene adipate) was not affected by the addition of rutile TiO 2 nanoparticles coated with a silicon–aluminum composite [ 60 ]. On the other hand, Antunes et al [ 71 ] demonstrated the effect of rutile TiO 2 nanoparticles on the accelerated weathering degradation of poly(lactic acid). The presence of TiO 2 nanoparticles not only improved the cold crystallization behavior of the PLA, acting as nucleating agents, but the nanoparticles also catalyzed the UV-initiated degradation of the PLA.…”

Effect of poly(ε‐caprolactone) and titanium (IV) dioxide content on the UV and hydrolytic degradation of poly(lactic acid)/poly(ε‐caprolactone) blends

“…As will be seen later in the discussion of the DSC results, after 2000 h degradation there are almost no PCL crystals left in the sample, while a reasonable number of PLA crystals have formed, that are probably visible as coalesced PLA spheres in the SEM picture of PLA/PCL/2000 h. The samples also became more yellow (Figure 1) because of the formation of chromophore groups (e.g., carboxyl, carbonyl, ketone, aldehyde, hydroxyl, ester, etc.) and conjugated double bonds in the polymers by the photo‐oxidative reactions during the accelerated weathering exposure 29 …”

“…The infrared spectrum of PLA presents the characteristic bands of the neat polymer. Since it is exactly the same polymer we used in our previous research, 29 we are not going to repeat the description of the spectrum here. The FTIR spectrum of neat PCL shows the absorption bands corresponding to the C═O stretching vibration at 1720 cm −1 and the CH 2 stretching at 2944 and 2863 cm −1 .…”

“…However, there are currently no reports on the weathering degradation of either PLA/PCL blends or PLA/PCL/TiO 2 nanocomposites. TiO 2 is very reactive and it is well‐known for its photocatalytic action, since the presence of these particles enhances the photodegradation of the polymer matrix 29 . Changes in PLA properties were observed after the PLA/TiO 2 nanocomposites were exposed to accelerated weathering up to 2000 h. It was also observed by Delgado‐Lima et al 30 when investigating PCL/TiO 2 under accelerated weathering for a period of 700 h, showing that the presence of titania enhanced polymer chain scission during UV exposure.…”