Recent Advances in Production of Ecofriendly Polylactide (PLA)–Calcium Sulfate (Anhydrite II) Composites: From the Evidence of Filler Stability to the Effects of PLA Matrix and Filling on Key Properties

Abstract

The melt–mixing of polylactide (PLA) with micro- and/or nanofillers is a key method used to obtain specific end-use characteristics and improvements of properties. So-called “insoluble” CaSO4 (CS) β-anhydrite II (AII) is a mineral filler recently considered for the industry of polymer composites. First, the study proves that AII made from natural gypsum by a specifically thermal treatment is highly stable compared to other CS forms. Then, PLAs of different isomer purity and molecular weights (for injection molding (IM) and extrusion), have been used to produce “green” composites filled with 20–40 wt.% AII. The composites show good thermal and mechanical properties, accounting for the excellent filler dispersion and stability. The stiffness of composites increases with the amount of filler, whereas their tensile strength is found to be dependent on PLA molecular weights. Interestingly, the impact resistance is improved by adding 20% AII into all investigated PLAs. Due to advanced kinetics of crystallization ascribed to the effects of AII and use of a PLA grade of high L-lactic acid isomer purity, the composites show after IM an impressive degree of crystallinity (DC), i.e., as high as 50%, while their Vicat softening temperature is remarkably increased to 160 °C, which are thermal properties of great interest for applications requiring elevated rigidity and heat resistance.

From Virtual Reconstruction to Additive Manufacturing: Application of Advanced Technologies for the Integration of a 17th-Century Wooden Ciborium

“…Usually, the most commonly used material for 3D printing is polylactic acid (PLA), a biobased, biodegradable and biocompatible thermoplastic polymer that is widely used in the food, medical, cosmetic, agro-industrial, textile, and art sectors, etc. [ 14 , 25 , 26 , 27 , 28 , 29 ]. Derived from renewable resources such as maize, it has a production capacity of over 140,000 tonnes/year and is hydrolytically degradable [ 30 , 31 ].…”

Engineering Polypropylene–Calcium Sulfate (Anhydrite II) Composites: The Key Role of Zinc Ionomers via Reactive Extrusion

“…It is important to note that CS is available in several forms: dihydrate—CaSO 4 ·2H 2 O (commonly known as gypsum), hemihydrate—CaSO 4 0.5H 2 O (Plaster of Paris, stucco, or bassanite), and different types of anhydrites [ 57 , 59 , 71 ]. The CS phases obtained by progressive dehydration and calcination of gypsum are in the following order [ 59 ]: dihydrate → hemihydrate → anhydrite III → anhydrite II → anhydrite I (the last one obtained at temperatures higher than 1180 °C).…”

“…In addition, we believe there is a misunderstanding related to the rapid water absorption or high moisture sensitivity that are specific to CS hemihydrate and to “soluble” β-anhydrite III (AIII), both derived from CaSO 4 ·2H 2 O (gypsum), as obtained by thermal treatments above 100 °C and at ca. 200 °C, respectively [ 57 ]. On the contrary, compared to these not stable forms, calcination of gypsum at higher temperatures (e.g., at 500–700 °C in an industrial process), allows us to obtain highly stable β-anhydrite II (AII), also known as “insoluble” anhydrite [ 57 , 58 , 59 ].…”

“…200 °C, respectively [ 57 ]. On the contrary, compared to these not stable forms, calcination of gypsum at higher temperatures (e.g., at 500–700 °C in an industrial process), allows us to obtain highly stable β-anhydrite II (AII), also known as “insoluble” anhydrite [ 57 , 58 , 59 ].…”

“…Consequently, further prospects are required to demonstrate its beneficial value with different polymer matrices and by considering the needs of a large range of applications. It is also important to note that AII has been used with promising results in the manufacturing of polylactic acid (PLA) composites even though this aliphatic (bio)polyester is known for its high thermal and hydrolytic sensitivity [ 57 , 60 ]. Accordingly, the use of natural gypsum, and particularly of its derivatives (i.e., AII), which are available in massive quantities, could be an interesting option to produce new PP-based composites and an answer to the current industrial requests.…”