Marius Murariu

High-Performance Polylactide/ZnO Nanocomposites Designed for Films and Fibers with Special End-Use Properties

Metallic oxides have been successfully investigated for the recycling of polylactide (PLA) via catalyzed unzipping depolymerization allowing for the selective recovery of lactide monomer. In this contribution, a metallic oxide nanofiller, that is, ZnO, has been dispersed into PLA without detrimental polyester degradation yielding PLA/ZnO nanocomposites directly suitable for producing films and fibers. The nanocomposites were produced by melt-blending two different grades of PLA with untreated ZnO and surface-treated ZnO nanoparticles. The surface treatment by silanization proved to be necessary for avoiding the decrease in molecular weight and thermal and mechanical properties of the filled polyester matrix. Silane-treated ZnO nanoparticles yielded nanocomposites characterized by good mechanical performances (tensile strength in the interval from 55 to 65 MPa), improved thermal stability, and fine nanofiller dispersion, as evidenced by microscopy investigations. PLA/ZnO nanocomposites were further extruded in films and fibers, respectively, characterized by anti-UV and antibacterial properties.

Photochemical Behavior of Polylactide/ZnO Nanocomposite Films

This study reports the effect of light on PLA/ZnO nanocomposites films produced by melt-extrusion. The attention focused on the discrimination between the photocatalytic degradation of PLA provoked by ZnO and the UV screening effect of the ZnO nanoparticles. The chemical modifications of PLA induced by UV light irradiation were analyzed using infrared spectroscopy and completed through the analysis of the low-molecular-weight photoproducts using IC and SPME and the characterization of chain scissions with SEC. A comprehensive mechanism for the photooxidation of PLA was then proposed. The results indicated that the photocatalytic activity of ZnO nanoparticles induces the oxidation of PLA. Because ZnO limits the penetration of light inside the samples, this effect mainly concerns the first micrometers at the surface of the exposed samples. Cross-sectional analysis using micro-IR and ATR-IR spectroscopies was performed to highlight the degradation profile in the PLA/ZnO nanocomposites.

Effect of Filler Content and Size on Transport Properties of Water Vapor in PLA/Calcium Sulfate Composites

Starting from calcium sulfate (gypsum) as fermentation byproduct of lactic acid production process, high-performance composites have been produced by melt-blending polylactide (PLA) and beta-anhydrite II (AII) filler, i.e., calcium sulfate hemihydrate previously dried at 500 degrees C. Characterized by attractive properties due to good filler dispersion throughout the polyester matrix and favorable interactions between components, these composites are interesting for potential use as biodegradable rigid packaging. The effect of filler content and mean particle diameter on the barrier properties such as sorption and diffusion to water vapor has been examined and compared to unfilled PLA. Even without additional treatments, the presence of the filler introduced constraints on molecular mobility in the permeable phase of amorphous PLA and the amount of solvent absorbed appears lower in the highly filled composites. Surprisingly, for PLA-30% AII compositions, by addition of filler characterized by high mean particle diameter (e.g., 43 microm) the thermodynamic diffusion parameter, D(0), decreased up to 2 orders of magnitude. The dimension of filler particles and their percentage in the continuous polymeric phase seem to be the most important parameters that determine the barrier properties of the PLA-AII composites to water vapor.

Recent advances in production of poly(lactic acid) (PLA) nanocomposites: a versatile method to tune crystallization properties of PLA

Abstract A new approach leading to poly(lactic acid) (PLA) nanocomposites designed with improved nucleating/crystallization ability has been developed. As proof of concept, nanofillers of different morphology (organo-modified layered silicates, halloysite nanotubes and silica) were surface-treated with ethylene bis-stearamide (EBS), a selected fatty amide able to promote chain mobility during PLA crystallization from the melt and nucleation. The fine dispersion of the nucleating additive via nanoparticles (NPs) as ‘nano-template’ is leading to nanocomposites showing unexpected improvements in PLA crystallization rate. This was evidenced by differential scanning calorimetry (DSC) from the high values of the degree of crystallinity (20–40%) with respect to neat PLA (4.3%) and the sharp decrease in crystallization half-time under isothermal conditions (at 110°C), even below one minute. Furthermore, after injection molding the outstanding crystallization properties of PLA were again confirmed. Accordingly, the PLA-nanofiller/EBS nanocomposites revealed remarkable degree of crystallinity (in the range of 30–40%). Surprisingly, the presence of EBS can significantly increase the impact resistance of PLA and PLA based nanocomposites. By considering the remarkable increasing in crystallinity, a key parameter to allow PLA utilization in durable applications, the development of the new approach is expected to lead to significant improvements in the processing and performances of PLA products.

Effect of ZnO nanofillers treated with triethoxy caprylylsilane on the isothermal and non-isothermal crystallization of poly(lactic acid)

The crystallization of PLA-silane surface-treated ZnO nanocomposites was investigated by DSC and compared to that of neat PLA. Several modes of crystallization were considered: isothermal and non-isothermal cold crystallization and also isothermal and non-isothermal melt crystallization. The kinetics of cold crystallization were studied using different methods, namely the Avrami and Ozawa-Flynn-Wall models, to calculate activation energies and kinetic constants. In contrast to what is typically observed when the foreign particles are added in a polymer matrix, the silane surface-treated ZnO delayed the crystallization of PLA and made it more difficult to start. The nucleation activity of the ZnO nanoparticles, ϕ, was calculated and found to be greater than 1 (ϕ = 1.7). This indicated that ZnO played an anti-nucleating role in the crystallization of PLA nanocomposites. This effect has been linked mainly to the interactions between the silane groups onto the surface of nanoparticles and PLA macromolecules. These interactions which reduce the mobility of polymer chains have been evidenced by rheological experiments.

Adhesion and micromechanical deformation processes in PLA/CaSO4 composites

PLA/CaSO4 composites were prepared from uncoated and stearic acid coated filler particles in a wide composition range. The strength of interfacial adhesion was estimated quantitatively with three independent methods. Structure was characterized by DSC, XRD and SEM measurements, while mechanical properties by tensile and instrumented impact tests. The results proved that adhesion is twice as strong in composites prepared with the uncoated particles than in those containing the coated filler. Coating changes also local deformation processes around the particles. Although debonding is the dominating micromechanical deformation process in all composites, local plastic deformation is larger around coated particles. The extent of this deformation depends very much also on the local distribution of particles. The final properties and performance of the composites depend unambiguously on the micromechanical deformation processes occurring during loading, on debonding and the subsequent plastic deformation. Stearic acid used for the coating of the filler seems to dissolve in the polymer and locally change its properties.

Key factors for tuning hydrolytic degradation of polylactide/zinc oxide nanocomposites

Abstract The hydrolytic degradation of thin films of polylactide/surface treated zinc oxide [poly(lactic acid) (PLA)/ZnOs] nanocomposites was investigated in phosphate buffer solution at the temperature of 37°C for more than 10 months. To produce PLA/ZnOs nanocomposites, the previously silanized metal oxide nanofiller has been dispersed into PLA by melt blending using twin screw extruders and the resulting dried pellets were shaped into thin films of about 70 μm thickness. For sake of comparison, pristine PLA was processed and investigated under similar conditions. The evolution of molecular weights of the PLA matrix, as well as of crystallinity and thermal parameters of interest, with the hydrolysis time, has been recorded by size exclusion chromatography (SEC) and differential scanning calorimetry (DSC), respectively. Accordingly, at longer hydrolysis time, the nanocomposites revealed better resistence to the hydrolytic degradation (lower weight loss, smaller decrease of molecular mass, no dramatic increase in dispersity), data that were also associated with the changes in the morphology of specimens over time as evidenced by visual analysis or by microscopy. The results show the possibility to tune the hydrolytic degradation and prolonging the service life of PLA throught the incorporation of a small amount of hydrophobic silanized nanofiller (ZnOs). A bulk degradation mechanism was assumed, whereas the delayed degradation of nanocomposites was ascribed to a slowdown of the water diffusion into PLA matrix thanks to important increases of the crystallinity and especially to the hydrophobic properties of ZnO nanofiller treated with ∼3 wt-% triethoxycaprylylsilane. Accordingly, the rate of hydrolytic degradation of PLA/ZnOs nanocomposite films can be reduced by increasing the loading of nanofiller and PLA crystallinity.

Calcium Sulfate as High-Performance Filler for Polylactide (PLA) or How to Recycle Gypsum as By-product of Lactic Acid Fermentation Process

Reinforcing of polylactide (PLA) with fillers can be an interesting solution to reduce its global price and to improve specific properties. Starting from calcium sulfate (gypsum) as by-product of the lactic acid fermentation process, novel high performance composites have been produced by melt-blending PLA and this filler after a previous specific dehydration performed at 500°C for min. 1 h. Due to PLA sensitivity towards hydrolysis, it has first been demonstrated that formation of β-anhydrite II (AII) by adequate thermal treatment of calcium sulfate hemihydrate is a prerequisite. Then, the modification of filler interfacial properties with different coating agents such as stearic acid (SA) and stearate salts has been considered. The effect of surface treatment on molecular, thermal and mechanical properties has been examined together with the morphology of the resulting composites. To take advantage of the improved lubricity and better wetting characteristics, the filler was coated by up to 2% (by weight) SA. The coating of the filler leads to PLA–AII composites that surprisingly exhibit thermal stability, cold crystallization and enhanced impact properties. Such remarkable performances can be accounted for by the good filler dispersion as evidenced by SEM–BSE imaging of fractured surfaces. As far as tensile proprieties are concerned, notable utilization of uncoated filler or filler coated by stearate salts leads to PLA–AII composites characterized by higher tensile strength and Youngs modulus values. The study represents a new approach in formulating new melt-processable grades with improved characteristic features by using PLA as polymer matrix.

Chapter 21 – Pathways to Biodegradable Flame Retardant Polymer (Nano)Composites

The realization of special formulations of biopolymers characterized by flame retardancy significantly enlarges their potential for utilization in engineering sectors while reducing the environmental impact. The production of nanocomposites using nanofillers of different morphologies, associated or not associated with traditional flame retardants (FR), is undeniably one of the most promising ways considered to improve the flame retardancy of biopolymers. The chapter presents current researches, results, and trends in the field of bionanocomposites (BNCs) characterized by improved fire resistance, with a special mention for polylactic acid (PLA), an aliphatic polyester that is derived from renewable resources and which is environmentally biodegradable. The chapter highlights the main developments in FR systems during the last decade using various nanofillers (clays, carbon nanotubes, graphite derivatives, etc.) and attempts in the formulation of innovative intumescent formulations. Furthermore, selected results are presented in relation to the characterization of novel BNCs with special end-use properties (flame retardancy) designed for the production of PLA fibers.

CHAPTER 4:Reactive Extrusion of PLA-based Materials: from Synthesis to Reactive Melt-blending

Reactive extrusion processing (REX) of polylactide (PLA) is reviewed with a focus on chemical aspects in order to modify the performances of PLA-based materials. The scope, advantages, limitations and specificity of various REX processes are examined in this context. The processes covered include: ring-opening polymerization (ROP) of lactide, free-radical grafting onto PLA backbone, coupling reactions and exchange reactions of PLA chains.