Novel Green Poly (Lactic Acid) – Silica Aerogel Composites for Thermal Insulation

The development of biodegradable polymers is showcased as one of the most effectiveinnovations in the polymer industry for addressing environmental issues,[1] and poly (lactic acid)(PLA) is one of the most promising environmentally friendly biodegradablepolymers, due to itseasy availability and favorable processability.[2] In fact, PLA is a thermoplastic bio-basedpolymer, produced from lactic acid via fermentation of agricultural products such as corn,cornstarch, or sugarcane.[2,3] It shows a relatively low thermal conductivity of 0.193 Wm−1K−1at ambient temperature. It also has a relatively light weight (density of 1.24 g/ml), a glass transitiontemperature of around 55–60°C and a low production cost. However, being derived from lacticacid, it is easily biodegradable, especially at high temperatures and high moisture levels, whichlimits its practical applications, confined until now mainly in the fields of filaments for 3D printingand disposable tableware and cutlery, even though applications in biomedicine (as a scaffoldmaterial for tissue engineering, in cosmetics or biomedical implants) are extensively studied andare growingly commercialized.[2,4]Silica aerogel is the most common type of aerogel.[5,6] Due to its unique properties, such as highspecific surface area (surface-to-volume ratio) (of 901m2/g), ultra-low density (0.365g/cm3) andhigh porosity (up to 99%+), it has received significant attention over the last few decades. One ofits main properties is a very low thermal conductivity, down to 0.013 Wm−1K−1 at roomtemperature and atmospheric pressure, with a further decrease to 0.010 Wm−1K−1 under vacuumat 300 K.[7] Being essentially a highly porous glass, it is also fully recyclable. In practicalapplications, it is usually treated to become hydrophobic in order to avoid the entrance of waterin its nanometric pores, which would rapidly decrease its thermal insulation properties.We present here preliminary studies over novel green composites prepared using PLA as thematrix and hydrophobized silica aerogel (SA) as the filler. In order to increase the resistance ofPLA towards atmospherical agents (i.e., rain and humidity) and temperatures, we explored thepossibility to crosslink the PLA chains using carbodiimidic compounds (CDI), which cause acontrollable crosslinking of the polymer chains, providing better mechanical properties andthermal stability, as also demonstrated by other researchers.[7] We hence prepared a ternarysolution (PLA + liquid solvent + SA), exploring different types of solvents (for example THF,Dioxane or DCM), different PLA/SA ratios and different amounts of CDI, from which all-solidsamples of appropriate sizes were produced. The samples have been characterized via FT-IR inorder to identify possible interactions between the hydrophobized silica functional groups and thePLA chains and to measure their thermal conductivities. The so-obtained results are presented and discussed.